Method for the preparation of planographic printing plates from silver images

ABSTRACT

A method for the preparation of a planographic printing plate wherein a sheet material comprising an outer hardenable hydrophilic colloid layer on the surface of which is concentrated a silver image which has been formed at that surface from complexed silver halide by silver complex diffusion transfer process, is treated with an aqueous lithographic fixer having a pH-value in the range from about 4.5 to about 7.2 and containing : 1. AN IRON(III) ammonium salt of an aliphatic dicarboxylic acid which at least superficially oxidizes said silver image 2. A PRECIPITATING AGENT FOR SILVER IONS, AND 3. AN ORGANIC NITROGEN-CONTAINING COMPOUND, WHICH REACTS WITH SAID OXIDIZED SILVER AT THE SURFACE OF THE SILVER IMAGE TO RENDER SAID IMAGE HYDROPHOBIC AND CONTAINS A STRUCTURAL PART CORRESPONDING TO THE FOLLOWING TAUTOMERIC STRUCTURE :   D R A W I N G

United States Patent 11 1 Serrien et al.

[451 Sept. 9, 1975 METHOD FOR THE PREPARATION OF PLANOGRAPHIC PRINTINGPLATES FROM SILVER IMAGES [73] Assignee: Agfa-Gevaert N.V., Mortsel,

Belgium [22] Filed: Oct. 3, 1973 [21] Appl. No.: 402,883

[30] Foreign Application Priority Data Oct. 4. l972 United Kingdom45792/72 [52] U.S. Cl 96/29 L; 96/33; 101/466 [51] Int. Cl.'-.. G03C5/54; G03F 7/02; B41M 5/00; B4lN 3/00 [58] Field of Search 96/29 L, 33,60 B, 60 F; l0l/466 [56] References Cited UNITED STATES PATENTS 3 0s3097 3/1963 Lassig ct al. 96/29 1. 3,490,906 ]/]970 Blake 96/29 L3,615,507 l0/l97l Bard 96/60 BF 3,676.125 7/[972 dcHaas.. 96/29 L 3 700450 10/1972 Cooley 96/60 BF 3,721,559 3/l973 DeI-Iaas et al. 96/29 LPrimary Examiner-Norman G. Torchin Assistant Examiner-Richard L.Schilling Attorney, Agent, or FirmWilliam J. Daniel ABSTRACT A methodfor the preparation of a planographic printing plate wherein a sheetmaterial comprising an outer hardenable hydrophilic colloid layer on thesurface of which is concentrated a silver image which has been formed atthat surface from complexed silver halide by silver complex diffusiontransfer process, is treated with an aqueous lithographic fixer having apH-value in the range from about 4.5 to about 7.2 and containing 1. an1ron(IlI) ammonium salt of an aliphatic dicarboxylic acid which at leastsuperficially oxidizes said silver image 2. a precipitating agent forsilver ions, and

3. an organic nitrogen-containing compound, which reacts with saidoxidized silver at the surface of the silver image to render said imagehydrophobic and contains a structural part corresponding to thefollowing tautomeric structure Hsi N 10 Claims, No Drawings METHOD FORTHE PREPARATION OF PLANOGRAPI-IIC PRINTING PLATES FROM SILVER IMAGES Thepresent invention relates to a method for the preparation ofplanographic printing plates by converting silver images intohydrophobic ink-receptive images and to liquid compositions forimproving the ink receptivity of the silver images.

It is known to produce planographic printing plates of improved qualityby the use of a sheet material comprising an outer hydrophilic colloidlayer, on the surface of which there is concentrated a silver image thathas been formed on this surface from complexed silver halide by a silvercomplex diffusion transfer process.

According to the method for the preparation of planographic printingplates described in the United Kingdom Patent Specification 1,241,661the thus prepared silver image is rendered hydrophobic with an aqueousliquid. called lithographic fixer, which liquid has a pH-value between 7and 12 and contains (1 hexacyanoferrateflll) ions. by means of whichsaid silver image is at least superficially oxidized, and (2) an organiccompound that is at least in part dissolved in said liquid and whichreacts with said oxidized silver to ren der said image hydrophobic.

Although the lithographic fixer described in the above-mentioned PatentSpecification has particularly favourable hydrophobizing properties somedisadvantages are associated therewith. For example thecyanoferrateflll) ion under the defined condition of a relatively highpH is not inert with respect to the mercaptocompounds, which are thepreferred compounds for the purpose cited under the above item (2).Indeed, as a consequence of the presence of the mercapto compound someof the hexacyanoferrate( III) ions become reduced and the thus formedcyanoferratefll) ions form a blue precipitate with the excesscyanoferrate(lIl) ions. Further, also during the preparation of theprinting plate by the oxidation of the silver image hexacyanoferrateUl)ions are formed, causing the formation of a blue precipitate and as aresult thereof a more or less pronounced staining of the imagebackground.

A further disadvantage associated with the use of a lithographic fixercontaining hexacyanoferrate(lll) ions resides in the fact that theseions have to be kept carefully out of contact of acids, for acids setfree the highly toxic hydrogen cyanide.

With reference to the relatively high pH (1 ll2) of the lithographicfixer special precautionary measures have to be taken for the operatingpersonnel and only highly alkali-resistant machine parts can be used.

It is an object of the present invention to provide a lithographic fixerof improved stability and presenting a good ink-receptivity to silverimages.

It has been found that said object can be attained by means of anaqueous lithographic fixer having a pH- value in the range of about 4.5to about 7.2 and containing:

1. an iron(lll) ammonium salt of an aliphatic dicarhoxylie acid asoxidizing agent for the silver image,

2. a precipitating agent for silver ions and 3. an organicnitrogen-containing compound having a thiol group or in its tautomericstructure a thione group as represented in the following tautomeriestructural parts:

capable of reacting with silver ions present at the surface of a silverimage and hydrophobizing the silver image.

In order to avoid that during the oxidation of the silver image with theorganic iron(III) ammonium salt, silver ions are accepted by the fixersolution and removed from the silver image portions so that they arelost for the hydrophobizing process, a compound is present in the fixersolution that keeps the silver ions formed at the place where they havebeen formed. For this purpose use is made of a substance thatprecipitates silver ions in the form of a poorly water-soluble silversalt.

A preferred precipitating agent is therefor a substance yielding iodideions e.g. potassium iodide, forming the very poorly soluble silveriodide.

A preferred lithographic fixer combination of the present inventioncontains in admixture with the above-cited compounds (1) to (3) analiphatic amino compound comprising at least 4 unbranched carbon atoms,preference being given to n-octylamine. The pH of the fixer is keptwithin the above-cited range and preferably at 6 to 6.2 by means of thedicarboxylic acid used for preparing the iron(IlI) ammonium salt thuspreventing the precipitation of Fe(OH) A suitable amount of aliphaticamino compound is in the range of 10 to ml per litre.

A preferred iron(IlI) salt for use according to the present invention isiron(III) ammonium malonate. A preferred lithographic fixer containsfrom 20 to g of iron(IlI) ions present in the form of iron(lll)ammoniummalonate.

Other suitable iron(lIl) ammonium salts are derived from the aliphaticdicarboxylic acids: oxalic acid, maleic acid and fumaric acid.

Preferred thione compounds correspond to the following structuralformulae:

l. Compound 1 is prepared as described in DTOS 1,946.263

nos

2. Compound 2 is prepared as described in BE-P 523,1 15

COOH

3. Compound 3 is prepared as described in BE-P 719,338

and comprise a sulphonic acid salt group for improving thewater-solubility of the tiol or thione compound.

The thione compound No. 4 which is the most preferred one can beprepared according to the following reaction scheme:

Preparation of compound 4 98 g of Z-hydrazino-6-sulfobenzothiazole aredissolved in 1.5 l of pyridine and 450 ml of water. 24 ml of carbondisulphide are added thereto and the resulting solution is refluxed for12 h. Subsequently, it is cooled to room temperature. The reactionmixture is poured out in 2 litres of water and acidified withconcentrated hydrochloric acid. The precipitate formed is sucked off andadded to 250 ml of water and 125 ml of N sodium hydroxide, so that theprecipitate is dissolved again. The resulting solution is concentratedby evaporation. The residue is added again to 1 litre of water and thesolution obtained is acidified again with concentrated hydrochloricacid. The precipitate is sucked off. washed until neutral, and dried invacuo at 110C. Yield: 57 g.

Other suitable thione-thiol compounds for use ac cording to the presentinvention belong to the class of imidazoline-Z-thiones andimidazolidine2-thiones and are e.g.

Preparation of compound 5 105 g of carbon disulphide are added dropwiseto a solution of 69.3 g of N-allylethylenediamine in 1200 ml of ethanolat a temperature of 50C. A white precipitate forms. Subsequently, themixture is refluxed for 20 h. During the reaction hydrogen sulphide isformed and a complete solution is obtained. The solution is concentratedby evaporation to a volume of 400 ml and then cooled down. Theprecipitate is sucked off and dried. Melting point: C. Yield: 46 g.

Preparation of compound 6 20.6 g of a-amino-isobutyric acid aresuspended in ml of ethanol. 12 g of potassium hydroxide in 12 ml ofwater are added thereto, whereupon 20 g of allylisothiocyanate dissolvedin 100 ml of ethanol are added also. The reaction mixture is refluxedfor 2 hours and then concentrated by evaporation. The residue is boiledup with 900 ml of 2N hydrochloric acid, filtered while hot, and cooled.The crystals are sucked off and recrystallized from a mixture ofmethanol and water over animal chorcoal. Melting point: 77C. Yield: 13g.

Preparation of compound 7 g of 85% purity potassium hydroxide, 66 g ofcarbon disulphide, 770 ml of ethanol, and l 15 ml of water are added to149 g of N-methyl-o-phenylenediaminedihydrochloride. The reactionmixture is stirred and refluxed for 3 hours while hydrogen sulphideescapes. Subsequently, the mixture is cooled down. The precipitateformed in the reaction mixture is dissolved again by the addition of 750ml of water. The solution is boiled with animal charcoal, filtered andadmixed at 70C with 64 ml of acetic acid and 120 ml of water. Thereaction mixture is cooled down and the resulting precipitate is suckedoff and recrystallized from ethanol. Melting point: C. Yield: 45 g.

Preparation of compound 8 91 g of thiosemicarbazide and 158 g ofcaprylic acid methyl ester are added to 23 g of sodium in 1 l ofanhydrous methanol. The reaction mixture is refluxed for 16 h andconcentrated by evaporation. The residue is dis- Solved in 200 ml ofwater by heating and cooled afterwards. The solid product is sucked offand the filtrate is acidified with 70 ml of acetic acid. During the cooling crystallization occurs. The crystals are sucked off, recrystallizedfrom 40% ethanol and washed with dichloroethane. Melting point: 190C.Yield: 92 g.

Preparation of compound 9 is described in FR-P Application 2,001,068.

Compound 10 is prepared as described in J.Gen.Chem.

Preparation of compound 1 1 50 ml of 40% t'ormol are added dropwise to amixture of 25.3 g of thiourea and 43 g of octylamine in 200 ml of waterwith stirring and cooling. 150 ml of ethanol are added as well and thestirring is continued for 16 h. Subsequently, the reaction mixture isconcentrated completely by evaporation, so that a white amorphousproduct is formed, which is recrystallized from ethanol and benzenerespectively. Melting point: 159C. Yield: 30 g.

Compound 12 is prepared as described in Can. l.Chem. 32, 59 (1954).

Preparation of compound 13 45.6 g of carbon disulphide are addeddropwise to a solution of 50 g of N-propylethylenediamine in 275 ml ofmethylglycol at a temperature of 40C. A white pre- 5 cipitate forms.Subsequently, the mixture is heated for 1 hour at 90C, so that hydrogensulphide is formed. The methylglycol is then evaporated partially invacuo and 500 ml of water are added to the residue. After 10 having beencooled the precipitate is sucked off and recrystallized from a mixtureof benzene and hexane. Melting point: 90C. Yield: 54 g.

15 14. Hooc Preparation of compound 14 see DOS 1,931 ,056.

These compounds and the compounds 1 to 3 are pre pared according totechniques known to those skilled in the art.

A relatively high amount of thione or thiol com- 5 pounds can beincorporated in the aqueous fixer when it contains water-misciblesolvents e.g. n-propanol or ethylene glycol monoacetate.

A suitable amount of organic thiol or thione com pound is in the rangeof 0.5 to 15 g per litre.

so I A preferred lithographic fixer according to the present inventionis composed as follows:

ir0n(|l|) salt 20 g to 120 g ss 5 N potassium iodide 5 40 g organicsolvenNs) npropanol 0 ml 200 ml ethylene glycol monoacetate 0 ml 150 mlwater to make 1 1 pH adjusted to 6-6.2 by means of malonic acid andammonium hydroxide.

The iron(lll) salt is present in the form of iron(lll) ammoniummalonate.

Optimum results were obtained by using a fixer solution prepared by thefollowing procedure: into 780 ml of an iron(lll)ammonium malonatesolution containing 10.4% by weight of iron(lll)ions 39 ml ofn-octylamine were dissolved whilst stirring. Subsequently after adding 1ml of n-propanol and ml of ethylene glycol monoacetate, 39 ml of a 10%by weight solution of the thione compound N0. 4 in highly concentratedaqueous ammonium hydroxide were added. The pH was adjusted to 5.0 bymeans of additional aqueous ammonium hydroxide whereupon 23 g ofpotassium iodide were dissolved in the obtained composition. Additionalaqueous ammonium hydroxide was added so as to reach a total liquidvolume of 1 litre and a pH of 6.1. The iron(lll)ammonium malonatesolution was prepared by dissolving 570 ml of iron(lll)chloride in waterin an amount sufficient to obtain a volume of 3 1. To the obtainedsolution 480 ml of ammonium hydroxide were added. The obtainedprecipitate was stirred for 2 h at room temperature and separated bysuction whereafter it was freed from chloride ions by washing withwater. Then the precipitate was redispersed in 3 l of water whereupon750 ml of malonic acid dissolved in 1500 ml of water were added.Stirring was continued for l h. The precipitate dissolved gradually inabout 24 h.

The green solution obtained was filtered and further diluted with waterup to a final volume of 7 l. The con centration of iron(lll) ions was104 g per litre.

The preparation of a planographic printing plate by using a fixeraccording to the present invention may be effected with a sheet materialcomprising a silver image obtained by any of the silver complexdiffusion transfer processes hereinafter described by way of example.

A first type of silver complex diffusion transfer process utilizes alight-sensitive material and a separate image-receiving materialconstituting the said sheet material and containing on an outer surfacethereof substance(s) for promoting the deposition of silver in the saidsilver complex diffusion transfer process.

In an embodiment of such a process the lightsensitive material comprisesa water-permeable hydrophilic colloid layer on top of the said silverhalide emulsion layer. Such a feature is described in the United KingdomPatent Specifications 869,190 998,955 and 998,956. Alternatively aspecial type of imagereceiving material can be used as described inUnited Kingdom Patent Specifications 1,013,344 and 1,054,252. In suchmaterials there may be incorporated in the image-receiving materialand/or light-sensitive materials substances that are essential or usefulfor carrying out the diffusion transfer image formation, e.g. developingagents, preservatives for these developing agents, complexing agents,stabilizers, alkaline substances, black-toning agents, hardeners andsoftening agents, so that the aqueous processing liquid need only be anaqueous solution of alkaline substances, or merely water, the lattertechnique being described in United Kingdom Patent Specification1,013,343. The incorporation of developing agents and preservativesthereof into the light-sensitive and/or image-receiving material fordiffusion transfer processes is described in United Kingdom PatentSpecifications 1,093,177 1,000,115 1,012,476 l.()42,477 1,054,253 and1,057,273. Embodiments in which hardening agents and more particularlylatent hardening agents are incorporated into the light-sensitive and/orimage'- receiving material for such processes are described in UnitedKingdom Patent specification 962,483 and German Patent Specification1,203,604.

A second type of silver complex diffusion transfer process utilizes asingle material. Such a single material comprises a silver halideemulsion layer and may comprise a hardenable hydrophilic colloid layer,either beneath or above such silver halide layer. Substances forbringing about the appearance of the diffusion transfer image(development nuclei) from the diffusing complexed silver halide may bepresent at an outer surface of the material or at an interface of thesaid layer(s). In the latter arrangement the silver image obtained bydiffusion transfer is obtained on top of hardenable hydrophilic colloidlayer and preferably is hydrophobized thereon.

According to a special embodiment the colloid layer with the silverimage obtained by diffusion transfer is transferred to another sheetmaterial, which may then be converted to a planographic printing platefor instance as described in the United Kingdom Patent Specification l,001 ,55 8.

The development nuclei may be supplied in a liquid medium to thehardenable hydrophilic colloid layer on top of the light-sensitivesilver halide emulsion layer. The development nuclei depositing at theouter surface of the said layer make that the diffusion transfer silverdeposition thus occurs at this outer surface. The silver image areas areconverted into ink-receptive printing areas to form a planographicprinting plate with the lithographic fixer of the present invention.

If desired, the development nuclei may be applied to the surface of thelight-sensitive recording material in the alkaline processing liquid forcarrying out the diffusion transfer image formation or from a separateliquid composition containing such nuclei which constitutes a step afterthe exposure of the light-sensitive material and before wetting with thealkaline processing liquid for forming the diffusion transfer image.

Substances for promoting the silver deposition from the diffusingcomplexed silver halide are sulphides of heavy metals such as thesulphides of antimony, bismuth, cadmium, cobalt, lead, nickel, silverand zinc. Other suitable salts are the selenides, polysulphides,polyselenides, mercaptans and tin(ll) halides. Heavy metals or theirsalts and fogged silver halides are suitable too. The complexed salts oflead and zinc sulphides are active both alone and when mixed withthioacetamide, dithiobiuret, and dithiooxamide. Heavy metals, preferablysilver, gold, platinum, palladium, and mercury may be used in theircolloidal form. The support of the photographic material comprising thesilver halide emulsion layer may be any conventional flexible supportsheet e.g. a paper sheet or a transparent hydrophobic film support, suchas a support of cellulose triacetate or of a polyester e.g. polyethyleneterephthalate.

The image sharpness of the silver pattern and, as a consequence thereof,the sharpness of the final result of printing can be improved byapplying antihalation dyes or pigments. These dyes or pigments may bepresent in the silver halide emulsion layer or in the support, butpreferably are incorporated into a layer situated between the silverhalide emulsion layer and the support. If a transparent support is used,the antihalation dyes or pigments, may be applied to the rear side ofthe material or on top of the emulsion layer dependent on the manner inwhich the exposure is carried out, viz. at the front side or through thesupport. Preferably a red or black antihalation dye or pigment is used.Any silver halide emulsion of the negative or direct-positive type maybe used depending on the nature of the original to be reproduced.Preferably emulsions of the negative type are used, having a somewhathigh sensitivity suitable for use in a camera. As a matter of fact,although the exposure of the silver halide emulsion layer may be carriedout according to any usual technique e.g. by contact, by backreflection, by transmission or episcopically. Usually an episcopicexposure in a camera is carried out, particularly when the developmentnuclei are provided on top of the silver halide emulsion layer asdetailedly described hereinafter. The silver halide emulsion layergenerally comprises an amount of silver halide equivalent to from 0.5g.sq. metre to 1.5 g sq. metre of silver nitrate and preferablyamounting only to the equivalent of about 1 g sq. metre of silvernitrate. This means a considerable economy of silver halide with respectto the silver halide content of emulsion layers commonly used in theproduction of diffusion transfer copies. If necessary a suitable subbinglayer is provided for strongly adhering the hydrophilic colloid layer(s)to the support sheet. The data in this paragraph also apply to anyembodiment for the production of silver pattern substantially at thesurface of a hydrophilic colloid layer according to the silver complexdiffusion transfer process described hereinafter.

The sheet materials suited for use with the fixer solution of theinvention are generally of simple composition. and may comprise asuitable support such as transparent hydrophobic film support or a papersheet, either in direct contact or indirectly, e.g., by means of asuitable subbing layer, with a hydrophilic colloid outer layer having atits surface a pattern of finely divided silver particles. A silverhalide emulsion layer may be present between the colloid outer layer andthe support and also antihalation dyes or pigments are provided. Thesilver pattern obtained by the complex diffusion transfer process may beintensified by chemical or physio-chemical after-treatment forintensifying the silver pattern in or at the surface of the outercolloid layer.

According to a preferred embodiment, a photographic material is usedwhich comprises successively a support layer (preferably a papersupport), an antihalation layer, a silver halide emulsion layer anddevelop ment nuclei on top thereof.

In order to obtain a diffusion transfer silver deposition that will besufficiently dense, the said photographic material has to be kept for aperiod of time, e.g. for about l seconds, in the dark after it has beenwetted with the alkaline processing liquid for carrying out thediffusion transfer image formation. The application of the lithographicfixer composition may take place during the period the multi-layermaterial is kept in the dark. However. this application of lithographicfixer may also occur thereafter.

For the production of the diffusion transfer silver image a photographicmaterial comprising development nuclei on top of the silver halideemulsion layer may be provided in roll-form. After image-wise exposure,e.g. an episcopic exposure in a camera or an exposure through atransparent original in contact with the photographic material, thelatter may be guided automatically through a usual processing unitcontaining the alkaline processing liquid and comprising guiding anddriving means as generally known in the art.

For the preparation of the hydrophilic colloid outer layer anyhardenable hydrophilic colloid is suitable. Although gelatin isfavoured, other hardenable hydrophilic colloids such as polyvinyl,alcohol, casein, carboxymethylcellulose and sodium alginate can be usedtoo, the nature of the hardener used being dependent on the type ofhydrophilic colloid to be hardened. lf gelatin is used for forming thehydrophilic colloid outer layer, it may be submitted to a treatment asdescribed in the United Kingdom Patent Specification 883,843 in order toimprove the printing characteristics of the planographic printing plate.

Hardening of the hydrophilic colloid layer may occur before, during orafter the treatment with the lithographic fixer composition and mustoccur at least to such an extent that no substantial amount of colloidis transferred on printing either to the rollers for applying water andink to the printing plate or to the material to be printed. In otherwords hardening (i.e. insolubilizing in water and strengthening againstmechanical damage) must occur at least to such an extent that thematerial obtained can be used as planographic printing plate.

The said hardening mostly occurs before the treatment with lithographicfixer. In that case the said hardening may be effected by addition ofthe generally known hardening agents for gelatin and similar col loids,such as formaldehyde, glyoxal, mucochloric acid and chrome alum, to thecoating composition of the outer layer, at the surface of which thesilver pattern will be produced, and/or to the coating composition ofanother layer, with which the said outer layer is in water-permeablerelationship whereby hardening of the said outer layer takes place bydiffusion of hardener from said other layer to said outer layer. When,as hereinbefore described a coating composition comprising developmentnuclei, possibly together with a minor amount of a hydrophilic colloidto keep the development nuclei in dispersion but insufficient to form acontinuous binder layer with the nuclei enclosed is applied on top ofthe said outer layer, the hardeners can also be incorporated into saidcoating composition. Hardening of the hydrophilic colloid binder of theouter layer may also occur during the production of the silver pattern.The said hardening may be effected by incorporating a latent hardenersin one or more layers of the sheet material, whereby a hardener isreleased at the stage of the application of an alkaline processingliquid for carrying out the complex silver diffusion transfer process.These latent hardeners are active only in a well defined pH-range,mostly the pH-range of the usual developing liquids. Finally, hardeningof the outer hydrophilic colloid layer can also occur after theproduction of the silver pattern namely by treatment with a hard eningliquid. This liquid may be an aqueous hardening composition appliedbefore the treatment with the lithographic fixer, the fixer compositionitself, or an aqueous hardening composition applied after the treatmentwith the said fixer. At least one compound for improving the hydrophilicproperties of the non-printing areas may be applied during thepreparation of the printing plate.

Thus the presence of certain hydrophilic colloid binders e.g.carboxymethylcellulose, gum arabic, sodium alginate, propyleneglycolester of alginic acid, hy-

droxyethyl starch, dextrine, hydroxyethylcellulose,polyvinylpyrrolidone, polystyrene sulphonic acid and polyvinyl alcoholin the outer hydrophilic colloid layer carrying at its surface thepattern of silver particles often improves the hydrophilic,ink-repellent proper ties of the non-printing areas of the printingplate finally obtained. Also hygroscopic substances e.g. sorbitol,glycerol, tri(hydroxyethyl)ether of glycerol and turkey red oil, andcertain wetting agents, may be present.

The hydrophilic colloid layer also may advantageously comprise a pigmentparticles homogeneously dispersed therein to prevent the so-calledscumming (i.e. ink-acceptance that arises in the non-printing areas ofthe printing plate after a certain number of copies has been printed).The usual inorganic pigments e.g. barium sulphate, titanium dioxide,china clay and silica applied from a colloidal solution, have proved tobe particularly suitable for this purpose. The pigment particles aregenerally homogeneously applied in such an amount that about 520 g sq.cmof the hydrophilic colloid outer layer are present. A similarantiscumming effect may also be obtained by adding at least one memberselected from colloidal silica, an inorganic acid eg o-phosphoric acid,a hygroscopic substance hereinbefore described and a suitable wettingagent to the fountain solution used during the printing process.Suitable wetting agents include:

sodium dodecylsulphate sodium tetradecylsulphate R-CH SO;,l\la wherein Rrepresents an alkyl group comprising from 14 to 18 carbon atoms Thelithographic fixer of the present invention is very stable to airialoxidation and to temperature fluctuations and it is suited for theproduction of planographic printing plates having non-staining imagebackground parts.

At the moment the treatment with the aqueous lithographic fixer startsthe outer colloid layer showing the silver pattern may be in a dry orwet condition. A superficial oxidation of the silver pattern suffices,al-

though complete oxidation is advantageous. Generally the treatment withthe lithographic fixer does not last long, mostly not longer than about20 seconds and can be accelerated by increasing the concentration of thecomponents in the said fixer. The plate may be stored for a long timebefore being fixed and even thereafter it may be stored for a long timebefore being used in the printing process. Preferably, however, thelithographic fixing step is carried out just before printing.

The lithographic fixer as well as the development or activating liquidfor the production of the diffusion transfer image can be applied indifferent ways, eg by spraying, by rubbing with a roller, or by dippingthe material to be treated in the liquid composition. The lithographicfixing step of the printing plate may proceed automatically byconducting the plate through a device having a narrow channel filledwith the fixer composition and conveying the printing plate at the endof the channel between two squeezing rollers removing the excess ofliquid.

The production of a silver image on top of the outer hydrophilic colloidlayer and the treatment with the fixed may occur in a compact processingunit comprising both the processing stations.

After the application of the lithographic fixer the sheet material isready for inking and use as a printing plate. Treatment of the materialwith a lacquer composition for strengthening the printing parts is notnecessary. Nevertheless, in some cases the hydropobic character of theink-receptive parts and their mechanical strength may be improved byapplying a lacquer thereon. Suitable lacquer compositions are solutionsof oils, waxes and resins in organic solvents. Suitable organic solventsare cyclohexanone, acetone, butanol, monomethyl ether of ethyleneglycol, monoethyl ether of diethylene glycol,tetrahydrothiophenel,1-dioxide, diacetone alcohol, dioxane,1,2-dichloroethane, ethyl acetate, trichloroethylene, butyl butyl-ate.diethanolamine and dimethylformamide. Mixtures of such organic solutionswith an aqueous phase or dispersions of such organic solutions in anaqueous phase are also suitable. In that case the aqueous phase maycontain thickeners or other compounds for improving the hydrophiliccharacter of the non-printing areas of the lithographic printing plateas described above. Suitable lacquers are described in the UnitedKingdom Patent Specifications 967,598 968,706 1,004,342 1,071,163 and1,071,164. Resins that have proved to be especially suitable forimproving the ink-receptive character of the printing areas andstrengthening them are phenol-formaldehyde resins e.g.phenolformaldehyde resins, a-eresol formaldehyde resins, andp-tert.-butylphenol formaldehyde resins, alkyd resins e.g. rosin maleicacid esters, epoxy resins. condensation productsof a poly( aryl ethyleneoxide) with an acid anhydride, an amine or another suitable compound andepoxidized polyesters.

The resin, wax or oil is usually used in a concentration of from aboutto about 500 g per litre of liquid lacquer composition. The liquidlacquer composition may be applied after the aqueous compositioncontaining the oxidizing agent has been applied and while the printingplate is still wet. When an emulsion lacquer is used the plate needs notnecessarily be wet at the stage the lacquer is applied. The lacquer maybe applied by dipping. spraying, spreading or by means of a materialsoaked therewith. The lacquer is rubbed e.g. with a plugof wadding. Thehydrophobic solid substances settle on the image areas and improve thehydrophobic ink-receptive character thereof. At the same time themechanical strength of the printing areas is improved. The adherence ofthe lacquer to the printing areas may further be improved by heating theplate.

Instead of being applied by means of a separate aftertreatment of theprinting plate, the liquid lacquer composition may also be incorporatedwith the aqueous composition containing the iron(lIl) ammonium salt ofan aliphatic dicarboxylic acid and the organic compounds for convertingthe silver image in a hydrophobicink-receptive pattern. The ratio of theorganic phase that contains the hydrophobic solid substances to theaqueous phase is generally between 1:] and 1:10.

The printing plate has to be wet at the stage the greasy printing ink isapplied. This is generally known in the art and it is usual to apply anaqueous liquid before applying the printing ink. This may occur by meansof a wet sponge or by means of the fountain arrangements (dampingsystem) of the printing machine.

The following examples illustrate the use of the lithographic fixeraccording to the present invention. All percentages are by weight unlessotherwise stated.

EXAMPLE 1 water 890 ml 12.5 aqueous solution of saponin l() ml aqueousdispersion of colloidal nickel sulphidc comprising per H1O ces 0.2 g ofnickel sulphide and l() g of gelatin as protective colloid l()() ml Thematerial obtained was exposed to an original and treated for 30 sec. inthe following processing composition:

sodium hydroxide l() g anhidrous sodium sulphite 75 g potassium bromidel g h \dr'oquinone 16 g lphenyl-3pyrazolidinone l g water up to 1000 mlanhydrous sodium thiosulphate l() g The material was then rubbed forsome 20 seconds with a plug of wadding saturated with the following 8lithographic fixer composition:

iron( lll )salt 74 g n-octylamine 39 ml n-propanol ll5 ml ethylene gl\col monoacetate 20 ml S Nii( TH NC% 4 g potassium iodide 23 g aqueoussolution of ammonium hydroxide to reach a final volume of l litre ofComposition having a 'pH of (1.00

The iron(lll) salt is present in the form of iron( 111) ammoniummalonate.

The material thus obtained was a positive planographic printing platewith very good printing characteristics and with which more than 1000copies of high quality could be printed. Applying a lithographic lacquerin order to strengthen the printing parts was not necessary and could beomitted. The fountain solution used on printing could be water or liquidhaving the following composition:

water ml glycerol 10 ml colloidal silica l ml phosphoric acid 2 mlEXAMPLE 2 To a paper support of weight 250 g per sq.m a highsensitivenegative silver ehlorobromide gelatin emulsion layer, hardened by meansof formaldehyde and comprising hydroquinone and 1-phenyl-3-pyrazolidinone, was applied so that per sq.m were present: an amount ofsilver halide equivalent to l g of silver nitrate, 0.5 g of hydroquinoneand 0.25 g of l-phenyl-3-pyrazolidinone.

The light-sensitive material obtained was image-wise exposed and thendipped for 10 sec. in the following processing liquid:

water 1000 ml sodium phosphatel2-water 75 g anhydrous sodium sulphite 40g potassium bromide 0.5 g anhydrous sodium thiosulphatc 10 g aqueousdispersion of colloidal nickel sulphide of example 1 20 ml In this way apositive diffusion transfer silver image was produced at the surface ofthe silver halide emulsion layer.

The material was then rubbed for some 15 seconds with a plug of waddingsaturated with the lithographic fixer described in Example 1, wherein,however, thione compound 4 had been replaced by a same amount ofmercapto compound I. The printing plate thus prepared could be used forprinting in the same manner as described in Example 1 and about the samegood results were obtained.

EXAMPLE 3 To a paper support of weight 135 g per sq.m; a highsensitivesilver chlorobromide gelatin emulsion layer comprising hydroquinone andl-phenyl-3- pyrazolidinone was applied so that per sq.m are present, anamount of silver halide equivalent to l g of silver nitrate, l g ofhydroquinone and 0.5 g of l-phenyl- Il-pyrazolidinone.

After drying the silver halide gelatin emulsion layer the latter wasovercoated in a proportion of g per sq.m with the following composition:

water 926 ml gelatin lo g l2.5 '71 aqueous solution of saponin 8 ml 20/r aqueous solution of formaldehyde 56 ml A Then the following liquid isapplied in a proportion of 22 g per sq.m:

water 890 ml 12.5 aqueous solution of saponin ml aqueous dispersion ofcolloidal nickel sulphide of Example 1 100 ml The light-sensitivematerial obtained was exposed image-wise and then guided through anautomatic twobath processing device, the baths of which had thefollowing compositions respectively:

I. (Activating liquid) water 1000 ml sodium phosphate-l2-water 75 ganhydrous sodium sulphite 40 g potassium bromide 0.5 g anhydrous sodiumthiosulphate 10 g I]. The lithographic fixer described in Example 1,with the modification that thione compound 4 had been replaced by a sameamount of the thiol compound listed herein.

The material obtained after this treatment was readly for use asplanographic printing plate with positive image values compared to theoriginal. More than 1000 copies could be printed therewith. The fountainsolution might be mere water or the specific fountain com position ofExample 1.

EXAMPLE 4 To a strong paper support of weight 135 g per sq.m a thin grayantihalation was applied by pouring a dispersion of 3 g of lamp black in1 litre ofa 4% aqueous solution of gelatin at a rate of 1 litre per 40sq.m.

To this antihalation layer a high-sensitive silver chlorobromide gelatinemulsion layer comprising hydroquinone and 1-phenyl-3-pyrazolidinone wasapplied in such a way that per sq.m. were present: an amount of silverhalide equivalent to l g of silver nitrate, l g of hydroquinone and 0.5g of l-phenyl-3-pyrazolidinone.

After drying. the silver halide gelatin emulsion layer was overcoated ata rate of 100 g per sq.m with the following composition:

water 926 ml gelatin 10 g 12.5% aqueous solution of saponin 8 ml )2aqueous solution of fomialdehyde 56 ml Then the following liquid wasapplied at a rate of 22 g per sq.m:

water 890 ml 12.5 71 aqueous solution of saponin 10 ml aqueousdispersion of colloidal nickel sulphide comprising per 100 ml of 0.2 gof nickel sulphide and 10 g of gelatin 100 ml The light-sensitivemultilayer thus obtained was image-wise exposed to an original.

This exposure might be an episcopic exposure in a camera or an exposurethrough an intermediate original having a sufficiently transparentbackground and being pressed with its rcarside against the nuclei sideof the multilayer material.

The multilayer material can be used in roller form and after theimage-wise exposure be cut off as desired.

After the image-wise exposure the multilayer material was guided througha processing unit containing the following alkaline liquid:

water 1000 ml sodium phosphate-l2-water g anhydrous sodium sulphite 40 gpotassium bromide 05 g anhydrous sodium thiosulphatc 10 g Thereafter themultilayer material was kept for some 10 seconds in the dark in order topermit an intense diffusion transfer silver deposition to take place. Asharp bronzed legible diffusion transfer image became visible on a graybackground.

The plate was then moistened (e.g. by means of a plug of wadding) forsome 20 seconds with the following lithographic fixer composition:

iron(lll) salt 60 g noctylamine 42 g n-propanol 100 ml ethylene glycolmonoacetate 100 ml S N NaO S Cl 1TH 6 g NC=S potassium iodide 30 gaqueous ammonium hydroxide solution of to reach a final volume of llitre of composition having a pH of 6.1.

The iron(lll) salt is present in the form of iron(lll) ammoniummalonate.

The said fixer composition was very stable to aerial oxidation and totemperature fluctuations. With 1 litre thereof about 100 printing plates(21 cm X 29.7 cm size) could be prepared.

The preparation of the printing plate proceeded automatically in acompact processing unit comprising the successive treating stations andwherein the plate was transported automatically from one unit to theother.

The printing plate thus obtained was ready for use as a planographicprinting plate with positive image values based on the original. It wasmounted on an offset apparatus and after having been wetted with wateror other fountain solution (e.g. a liquid composed of ml of water, 10 mlof glycerol, 2 ml of colloidal silica and 2 ml of phosphoric acid), thefatty printing ink was applied and printing started. More than 1000copies were printed with the plate thus prepared.

We claim:

1. A method for preparing a planographic printing plate wherein a sheetmaterial comprising an outer hardenable hydrophilic colloid layer on thesurface of which is concentrated a silver image which has been formed atthat surface from complexed silver halide by silver complex diffusiontransfer process is contacted with an aqueous lithographic fixer havinga pH-value in the range from about 4.5 to about 6.2 and containing:

1. an iron(lll) ammonium salt of an aliphatic dicarboxylic acid which atleast superficially oxidizes said silver imgage,

2. a compound yielding iodide ions for precipitating said silver ions,and

3. an organic heterocyclic nitrogen compound having in a 5- orfi-membered ring a thione group corre sponding to the followingtautomeric structure:

for rendering said superficial oxidized silver image hydrophobic.

2. A method according to claim 1, wherein said lithographic fixercontains as said iron( III) salt from 20 to 120 g of iron(lll) ammoniummalonate per litre.

3. A method according to claim 1 wherein said heter ocyelic nitrogencompound includes a sulphonic acid group in free acid or salt form.

4. A method according to claim 1, wherein said heterocyclic compound hasthe following structural for mula:

s N c \NH Qatari 7. An aqueous lithographic fixer according to claim 6wherein said heterocyclic nitrogen compound includes a sulfonic acidgroup in free acid or salt form.

8. An aqueous lithographic fixer according to claim 6, wherein theheterocyclic nitrogen compound corresponds to the following structuralformula:

9. An aqueous lithographic fixer according to claim 6, wherein saidfixer contains a water-miscible solvent that improves the dissolving ofthe mercapto or thione compound in the fixer.

10. A lithographic fixer according to claim 6 of the followingcomposition:

iron(lll) salt in the form of iron(lll) ammonium malonate 20 g to 120 gS\ /N\ Naons TH 0.5 gto 15 g \/U NC=S n-octylaminc H) g to ml potassiumiodide 5 g to 40 g organic solvcnfls) n-propanol (1 ml to 200 mlethylene glycol monoacctate 0 ml to [50 ml water to make I 1 pH adjustedto 6-6.2 by means of malonic acid and ammonium hydroxide.

1. AN IRON(III) AMMONIUM SALT OF AN AIPHATIC DICARBOXYLIC ACID WHICH ATLEAST SUPERFICIALLY OXIDIZES SAID SILVER IMGAGE,
 1. A METHOD FORPREPARING A PLANOGRAPHIC PRINTING PLATE WHEREIN A SHEET MATERIALCOMPRISING AN OUTER HARDENABLE HYDROPHILIC COHOID LAYER ON THE SURFACEOF WHICH IS CONCENTRATED A SILVER IMAGE WHICH HAS BEEN FORMED AT THATSURFACE FROM COMPLEXED SILVER HALIDE BY SILVER COMPLEX DIFFUSIONTRANSFER PROCESS IS CONTACTED WITH AN AQUEOUS LITHOGRAPHIC FIXER HAVINGA PH-VALUE IN THE RANGE FROM ABOUT 4.5 TO ABOUT 6.2 AND CONTAINING:
 2. ACOMPOUND YIELDING IODIDE IONS FOR PRECIPITATING SAID SILVER IONS, AND 2.a compound yielding iodide ions for precipitating said silver ions, and2. A method according to claim 1, wherein said lithographic fixercontains as said iron(III) salt from 20 to 120 g of iron(III) ammoniummalonate per litre.
 2. a compound yielding iodide ions for precipitatingsaid silver ions, and
 3. an organic heterocyclic nitrogen compoundhaving in a 5- or 6-membered ring a thione group corresponding to thefollowing tautomeric structure:
 3. AN ORGANIC HETEROCYCILE NITROGENCOMPOUND HAVING IN A 5- RO 6- MEMBERED RING A THIONE GROUP CORRESPONDINGTO THE FOLLOWING TAUTOMERIC STRUCTURE:
 3. A method according to claim 1wherein said heterocyclic nitrogen compound includes a sulphonic acidgroup in free acid or salt form.
 3. an organic heterocyclic nitrogencompound having in a 5- or 6-membered ring a thione group correspondingto the following tautomeric structure:
 4. A method according to claim 1,wherein said heterocyclic compound has the following structural formula:5. A method according to claim 1, wherein the said lithographic fixercontains from 0.5 to 15 g of said organic thiol or thione compound perlitre.
 6. An aqueous lithographic fixer having a pH generally in therange of about pH 4.5-6.2 containing:
 7. An aqueous lithographic fixeraccording to claim 6 wherein said heterocyclic nitrogen compoundincludes a sulfonic acid group in free acid or salt form.
 8. An aqueouslithographic fixer according to claim 6, wherein the heterocyclicnitrogen compound corresponds to the following structural formula:
 9. Anaqueous lithographic fixer according to claim 6, wherein said fixercontains a water-miscible solvent that improves the dissolving of themercapto or thione compound in the fixer.
 10. A lithographic fixeraccording to claim 6 of the following composition: